Product that is hot rolled into long steel and use thereof

ABSTRACT

The present invention refers to a hot rolled long steel product (wire rod and bar) of ultra-low carbon steel (UBC), interstitial-free, for use in drawn wires and rods. In this way, the product obtained through the hot rolling process of long steels using IF-type ultra-low carbon steel has different mechanical features, such as: high capacity of cold-forming, high capacity of area reduction in the tensile assay (87-94%), improved limit of resistance and outflow limit and high drawing capacity.

FIELD OF THE INVENTION

The present invention refers to a hot rolled long steel product (wire rod and bar) of ultra-low carbon steel (UBC), interstitial-free, for use in drawn wires and rods. In this way, the product obtained through the hot rolling process of long steels using IF-type ultra-low carbon steel has different mechanical features, such as: high capacity of cold-forming, high capacity of area reduction in the tensile assay (87-94%), improved limit of resistance and outflow limit and high drawing capacity.

BACKGROUND OF THE INVENTION

Non-flat or long steel have various profiles, namely, structural profiles type I, U, T, angle brackets, flat, square or round bars, tubes, wires, wire rod, etc. For producing steel in the steel industry, many metallurgical processes are required to obtain the final product that will be marketed as a plate, a tube, a round bar, a profile, etc. After refining the pig iron by processes such as the Bessemer, Siemens-Martin, LD oxygen converters or in Electrical Furnaces (melting of offscourings), the appropriate amounts of elements that will form the final allow are added. Once the steel alloy is settled in due proportions, the liquid metal is poured in conventional ingot moulds or by the sequential method (more modern) known as continuous casting.

Due to the need to keep up with new manufacturing processes, where the equipment increasingly undergo steel to greatest deformation associated with high rates of deformation, the long steel segment of the steel industry has been studying alternatives in the development of steels for meeting these continuous demands of the steel processing industry.

In the metallurgy of long steel, the main hot rolled products are tubes, profiles, rails, bars, rods and wire rod. The wire rod, for its turn, is the main product that undergoes beneficiation processes before its final application, generating products such as drawn wires, bolts, nuts, rivets, screens and artifacts, in addition to several other applications.

Steels with low percentages of interstitial elements are used in applications of high formability in deep embossing. The first steels used to meet such conformability requirements were low-carbon steels and steels with low content of impurities. However, with the challenge of producing increasingly complex shapes by cold forming, at the decade of 1970 research and development of steels more flexible and soft than conventional steels, and thus, with higher formability and drawability, begun. These researches led to the understanding that residual elements in solution in the body centered structure (CCC) of ferrite would act on the crystallographic texture, which, by its turn, macroscopically reflects on drawability of steels.

It was further verified that other alloying elements influence the recrystallization and segregation to grain boundaries. Then, the IF (interstitial-free) steels were developed, since the low-carbon steels had low surface quality and were hard to be shaped due to the presence of interstitial elements in its structure. The acronym “IF” expresses the lack of carbon and nitrogen elements in the interstices of the ferritic structure of steels, which are removed from the state of solid solution and may form precipitates by addition of elements that form carbide and nitride, such as titanium and niobium, thus leaving a steel free of interstitial elements.

The IF steels are a class of steels different from conventional steels by the following properties: low outflow yield, high surface quality and great total elongation. Such properties are possible by the control of interstitial elements, such as carbon and nitrogen.

Soon, the production and processing of IF steels requires steps that minimize the presence of carbon and nitrogen in solution. Additionally to the technological advances in the vacuum degassing techniques, which enabled the steel mill to produce steels with increasingly low levels of carbon and nitrogen—smaller than 25 to 30 ppm or 0.0025 to 0.0030% each—the industrial development of IF steels took place due to the evolution of two lines of development in the steel industry: forming steels, with the fixation optimization of interstitial elements in precipitated carbides and nitrides, and micro-alloyed steels, with the use of titanium and niobium in the phase formation that can precipitate in these steels and removal, at maximum possible extension, of the interstitial elements of the solid solution.

Mechanically, IF steels have high drawability and formability, since without the interstitial elements, these steels are malleable and soft.

The IF steel is widely used in cold-rolled flat products, in the automotive segment.

The international patent application number WO/2010/049950, filed on Oct. 27, 2009, under the title “Production of ultra-fine grains in interstitial free (if) steels by equal channel angular extrusion (ECAE)”, describes interstitial-free steels (IF) featuring high resistance and ductility, and a process for producing said steel. Particularly, the invention relates to a process for producing ultra fine grains in interstitial-free steels by channel angular extrusion (ECAE) to produce long products on large scale. It is a document where an IF steel (Ti stabilized) is used, but in an extrusion process, named ECAE, aiming at the production of ultra fine grains with high mechanical resistance. The large-scale production through this extrusion device (ECAE) used is unknown, and thus, it is understood that only experiments on specimens of IF steel have been conducted. Moreover, it was not cited in what kind of long steel processing it could be applied. And finally, this document indicates that all processing has been performed at cold conditions, and therefore, the mechanical properties obtained are very different from hot-rolled products.

The Brazilian patent application number PI 0605810-8, filed on Dec. 22, 2006, under the title “Composition of low carbon steel for electrical conduction and resulting low carbon steel which can be conformed by flat and non-flat rolling” describes a composition of low carbon steel for electrical conduction purposes, which can be conformed by flat and non-flat rolling. More particularly, it is a low carbon steel that can be hot or cold rolled, whose chemical composition comprises the main chemical elements found in common steel products: carbon, manganese, silicon, sulphur, phosphorus and aluminum, with the following maximum limits (expressed in parts per million and percent by weight) of the main chemical elements: C<0.08%; Mn<0.3%; S<0.05%; P<0.04%; and Al<0.04%. It is a patent application for a low-carbon steel with low electrical resistivity characteristics, showing characteristics of mechanical properties quite different from those of the present invention.

The Chinese patent application No. CN103469061, filed on Sep. 6, 2013, under the title “Ultra-low carbon steel wire rod for electric conduction and production method copies thereof, describes a steel wire rod of ultra-low carbon steel for electrical conduction and a method of producing the same. The ultra low steel wire rod for electrical conduction presents the following chemical composition: 0.002 to 0.005% of C, 0.004 to 0.010% of Si, 0.05 to 0.15% of Mn, 0.015% of P or less, 0.010% of S or less, 0.0050 to 0.0080% of total oxygen and 0.0050% or less of total aluminum. The electrical conductivity of the wire rod disclosed by the invention is greater than or equal to 15%, the tensile strength is less than or equal to 300 MPa, and the area reduction index is greater than or equal to 80%, and the wire rod produced can partially replace the copper conductive materials, thus reducing the production cost. This is a patent application for an ultra low carbon steel with low electrical resistivity characteristics, wherein the mechanical properties of IF steel of the present invention are quite different.

The Korean patent application No. KR20040091281, filed on Apr. 21, 2003, under the title “Method for ultra-low carbon steel sheet manufacturing by electric field heat treatment”, discloses a method for manufacturing ultra-low carbon steel sheets by heat treatment of the electric field, in order to increase flexibility and reduce the cost of manufacturing through a process of final annealing under an electric field imposed when producing the ultra-low carbon steel sheet. The method for manufacturing of ultra-low carbon steel sheets is carried out through the process of cold rolling and heat treatment of castings, the method comprising a step of cold rolling and a step of heat treatment of electric field, in which the annealing process is performed at a temperature of 650-800° C. for 10 to 20 minutes. Thus, this is a patent application related to flat steel, for a method/process of heat treatment and cold processing of an IF steel.

Thus, as it can be observed, no prior art document describes or suggests a hot rolled product in interstitial-free, ultra-low carbon long steel (wire rod and bar) for use in drawn wires.

SUMMARY OF THE INVENTION

It is an object of the present invention providing a hot rolled long steel product comprising interstitial free, ultra-low carbon steel, the steel being stabilized by titanium and comprising the following weight percentage ranges: 0.0010% to 0.0055 of C, 0.0020 to 0.0060% of N, 0.0040 to 0.025% of S and 0.040 to 0.100% of Ti.

It is also an object of the present invention providing a hot rolled long steel product comprising interstitial free, ultra-low carbon steel, the steel being stabilized by titanium and niobium and comprising the following weight percentage ranges: 0.0010% to 0.0055 of C, 0.0020 to 0.0060% of N, 0.0040 to 0.025% of S, 0.010 to 0.060% of Ti and 0.010 to 0.050% of Nb.

It is also an object of the present invention providing the use of said hot rolled long steel product for use in drawn wires, bolts, nuts, rivets, pins, rods, sieves and artifacts.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure and operation of the system of the instant invention, along with the additional advantages thereof, can be better understood with reference to the attached drawings and to the description that follows:

FIG. 1 illustrates results proven by a hot lamination metallographic assay, comparing the UBC/IF steel the present invention with a low carbon steel (SAE 1005) of high quality, from LD steel mill and the vacuum refining process in the wire drawing process.

FIG. 2 illustrates the evolution of tensile strength obtained in each drawing pass and its respective reduction for drawing pass.

FIG. 3 illustrates the evolution of tensile strength obtained in each drawing pass and its respective area reduction reported in the tensile assay.

FIG. 4 shows that the hot rolled IF steel, applied in the production of long steel, features superior conformability feature compared to commercially available steels.

FIG. 5 shows the processing flow of materials.

DETAILED DESCRIPTION OF THE INVENTION

Although this invention can be susceptible to different embodiments, preferred embodiments are shown in the drawings and in the following detailed discussion, with the understanding that this description should be considered an example of the principles of the invention without the intention of limiting the present invention to what has been illustrated and described herein.

The main approach of this invention is related to a hot rolled interstitial-free (IF), ultra-low carbon (UBC) long steel product (wire rod and bar), for use in drawn wires, showing different mechanical characteristics, such as cold-forming ability, high ability of area reduction in the tensile assay and improved strength limit and outflow limit.

In a preferred embodiment of the instant invention, the hot rolled long steel product comprises interstitial free, ultra-low carbon steel, the steel being stabilized by titanium and comprising the following weight percentage ranges: 0.0010% to 0.0055 of C, 0.0020 to 0.0060% of N, 0.0040 to 0.025% of S and 0.040 to 0.100% of Ti.

In another preferred embodiment of the instant invention, the hot rolled long steel product comprises interstitial free, ultra-low carbon steel, the steel being stabilized by titanium and niobium and comprising the following weight percentage ranges: 0.0010% to 0.0055 of C, 0.0020 to 0.0060% of N, 0.0040 to 0.025% of S, 0.010 to 0.060% of Ti and 0.010 to 0.050% of Nb.

It should be emphasized, in this regard, that for elements C, N and S, the smallest possible amount present in the steel is better, since these elements hinder the obtainment of IF steel.

In another preferred embodiment of the instant invention, said hot rolled long steel product is applied in: drawn wires); b) hot-rolled wire rods and bars of any size and cross-section geometry; and c) drawn wires and bars of any dimension and cross-section geometry.

Said hot rolled long steel product can be also applied in products derived from the products listed above, such as pins, rods and fasteners selected from the group consisting of screws, nuts, nails, staples and rivets.

The hot rolled long steel product of this invention presents an area reduction value in uniaxial tensile assay (yield coefficient) between 87% and 94%, i.e., a range that cannot be guaranteed by conventional ultra low carbon steel.

The area reduction values obtained in the uniaxial tensile assay of IF steel of the present invention denote a material with cold plastic deformation capacity not available in the long steel niche. With this steel technology, cold reductions can be obtained that have not been yet achieved with the currently available steels. Performance tests have shown ability to cold reducing IF steel, in the drawing process, by 99.4%.

With this ability of cold deforming IF steel wire rod, wires that currently require intermediate heat treatment processes are no longer needed in some cases, contributing to cost reduction in the wire production chain.

Moreover, besides of the superior drawing capacity, IF steel enables the cold production of complex parts with high degree of plastic deformation.

It is also important to point out that the material of the present invention has low electrical resistivity characteristics, in which the measurements point to around 0.130 Ωmm2/m (20° C.).

Comparing the mechanical properties of UBC/IF steel with a low carbon steel (SAE 1005) in wire drawing process after hot rolling process, the results have been observed as described in table 1 below:

TABLE 1 Stretching Area Reduction LE LR Hardness Sample (%) (%) (MPa) (MPa) (HRB) SAE 1005 38 82 258 328 63 IF 41 93 190 282 42

Moreover, the results of area reduction recorded in the tensile tests after every drawing pass indicate that the IF steel of the present invention has additional capacity of deformation in the drawing process when compared to SAE 1005 steel.

Additionally, the values of mechanical properties obtained for IF steel signal the possibility of operation with higher rates of deformation by drawing pass, when compared to SAE 1005 steel.

As it can see observed through the results of the assays presented in FIGS. 1 to 4, the IF steel of the present invention demonstrated excellent characteristics for application in market segments that require severe cold deformation. And then, according to the great capacity of cold deformation observed for IF steel, it is indicated that this can be applied to replace some steels that require intermediary heat treatment to ensure additional reductions.

Lastly, it is worth noting that the production process of the hot rolled product of the present invention eliminates the annealing steps, providing cost reduction of the final production. The steels currently available do not have the cold deformation ability that the instant invention presents, and intermediate heat treatment processing is required in some cases, for continuing with the cold forming process.

The proposed solution uses a steel with differentiated processing route in steel mill with LD converter and use of RH type vacuum degasser, producing an ultra low carbon steel with the addition of elements for stabilization of the remaining carbon and nitrogen content, for obtaining a steel free of interstitial elements (IF—Interstitial Free). Thus, this steel reaches cold forming properties superior to the existing/available steels for the cited applications.

EXAMPLE

An interstitial-free (IF) ultra-low carbon steel and a low-carbon steel (SAE 1005) were used, both provided by Companhia Siderúrgica Nacional (CSN). The manufacturing route for steel production includes LD steel mill and a vacuum degasser, the UBC steel being Ti stabilized. In table 2 the chemical compositions of steels which were used in this study are presented.

TABLE 2 Chemical composition of the materials used (% by weight). Steel C Mn P S Si Cu Nb Cr Al N Ti SAE 1005 .0530 .31 .011 .016 .008 .008 .003 .008 .052 .0034 .001 IF .0022 .11 .007 .007 .010 .014 .017 .018 .045 .0038 .064

The materials were made available in the form of sheets from continuous casting, which were cut in square blocks of 250 mm. The blocks were rolled in circular section bars of 12.70 mm to allow the subsequent drawing process.

The experimental procedures and methodologies were divided according to the following sequence, in which the following have been carried out: (1) drawing experiments of 12.70 mm gauge to the 5.50 mm gauge, (2) annealing heat treatment, (3) drawing of the 5.50 mm gauge to 1.25 mm gauge, (4) assays for experimental characterization and (5) metallographic analysis.

Taking as reference the hot rolling process for wire rod production, usually the smallest gauge available in the Brazilian market is 5.50 mm gauge, which is the most demanded for thin wire drawing processes. To keep the reference on this gauge, hot rolled bars obtained in the 12.70 mm section were drawn in 5.50 mm wires in 6 drawing passes for both steels under study, and later, in 10 passes for the 1.25 mm gauge, as it is shown in the sequence of FIG. 5.

The drawing equipment used for carrying out the assays was a vertical monoblock drawing mill containing a reduction pass, where the input material can be in the form of bar or roll, passing through a box containing solid soap (calcium stearate) having a tungsten carbide spinnerette coupled to its outlet, being then wrapped in traction block.

After each drawing pass, a sample with approximately 1 m length was withdrawn, enabling the execution of up to 3 uniaxial tensile assays, hardness measurements, cutting and preparation of samples for metallographic analysis. At every sample withdrawal, the material previously drawn was again pointed and, then, the tooling of the spinneret box was replaced for carrying out the next pass. Table 3 indicates the passes plan used to obtain the 5.50 mm gauge from the 12.70 mm gauge. Table 4 presents the passes plan used to obtain 1.25 mm gauge from 5.50 mm gauge.

TABLE 3 Passes plan of the first drawing step. 1st 2nd 3rd 4th 5th 6th Pass Pass Pass Pass Pass Pass 11.05 9.65 7.95 7.08 6.25 5.50 mm mm mm mm mm mm

TABLE 4 Passes plan of the second drawing step. 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass 4.50 3.73 3.12 2.65 2.27 1.97 1.73 1.53 1.38 1.25 mm mm mm mm mm mm mm mm mm mm

Heat Treatment

To meet the study proposal, heat treatments on 5.50 mm gauge of both steels were carried out in order to restore the mechanical properties of cold-deformed materials to the hot rolled state conditions. An oven of electrical resistance heating, manufactured by Brasimet, was used for carrying out of the assays, and the thermal treatments consisted of isothermal annealing of specimens at temperatures of 800 and 820° C. during 5 minutes, for BC and UBC steels, respectively.

The specimens used for the annealing assay were manually straightened with the aid of wood and a rubber hammer, as well as cut with adequate length to allow later tensile assay, hardness measurement and microstructural characterization.

In the study of mechanical behavior of materials subjected to drawing passes, uniaxial tensile assays and microhardness assays were carried out between the drawing passes, considering one sample for each drawing pass. In order to carry out the assays, a universal testing machine with load capacity of 250 kN, manufactured by INSTRON, model 5585H, with an extensometry video system was used, where the registration of the offset of the markings on the base of measurement is carried out with a camera, that is, without contact with the sample. The load cell used has 30 kN capacity and 10 N of accuracy. To characterize the hardness evolution in function of the hardening caused by the drawing passes, microhardness tests between the drawing passes were carried out, considering one sample for each pass.

In the microstructural characterization, the same samples prepared for carrying out the Vickers microhardness assay were used, where the polished surfaces suffered chemical attack with a 3% Nital solution during 10 s and Klemms attack (1 g of potassium metabisulfite and 50 ml of saturated sodium thiosulphate) for about 2 to 3 minutes for BC and UBC steels, respectively.

The isothermal annealing performed on the samples previously drawn on the 5.50 mm gauge presented satisfactory results in relation to the purpose of the test, since the mechanical properties and microstructure conditions showed values close to the material that started the drawing process in the 12.70 mm gauge. The results of the mechanical properties obtained in the samples after the annealing heat treatment are given in Table 7.

TABLE 7 Mechanical properties after annealing. LE LR Area reduction Stretching Hardness Steel (MPa) (MPa) (%) (%) (HRB) SAE 1005 258 347 81 38 60 IF 180 285 92 40 35

Considering the limit values of tensile strength obtained at each drawing pass and relating the respective value of area reduction due to drawing pass, FIG. 2 shows this relationship for each subsequent drawing pass. The hardening rate for a drawn wire is defined as the increase in the tensile strength limit given by the amount of area reduction after cold work.

Based on drawing experiments in the uniaxial tensile assays, hardness measurements and metallographic analysis laboratory carried out on samples of UBC and BC steels, it was possible to establish the following conclusions:

1) to achieve the same strength, UBC steel must be submitted to more hardening;

2) the relationship between the area reduction observed in the tensile assay and the material resistance has a linear relationship;

3) UBC steel had higher cold deformation capacity in comparison to BC steel;

4) UBC steel has lower hardness values than BC steel, showing an average difference of hardness on each pass of the order of 45 HV;

5) it was possible to establish a predictive model of drawn wire resistance in function of the drawing plan;

6) the UBC steel can be applied in fine wire drawing and the intermediary annealing steps can be eliminated in applications where steels with greater resistance are used.

Thus, although only some embodiments of the present invention have been shown, it will be understood that several omissions, substitutions and changes can be carried out by a person skilled in the art, without departing from the spirit and scope of this invention. The embodiments described should be considered in all respects only as illustrative and not as limitative.

It is expressly provided that all combinations of elements that perform the same function in substantially the same way to achieve the same results are within the scope of the invention. The substitution of elements in an embodiment described to another is also fully comprised and contemplated.

It should be also understood that the drawings are not necessarily in scale, and are only conceptual in nature. The intention is, therefore, to be limited, as indicated by the scope of the attached claims. 

1. Hot rolled long steel product, characterized in that it comprises interstitial-free, ultra-low carbon steel, wherein said ultra-low carbon steel is stabilized by titanium, and that the composition of said steel comprises the following percentage ranges by weight: 0.0010 to 0.0055% of C, 0.0020 to 0.0060% of N, 0.0040 to 0.025% of S and 0.040 to 0.100% of Ti.
 2. Product, according to claim 1, characterized in that it presents an area reduction value in the uniaxial tensile assay between 87% and 94%.
 3. Hot rolled long steel product, characterized in that it comprises interstitial-free, ultra-low carbon steel, wherein said ultra-low carbon steel is stabilized by titanium and niobium, and that the composition of said steel comprises the following percentage ranges by weight: 0.0010 to 0.0055% of C, 0.0020 to 0.0060% of N, 0.0040 to 0.025% of S, 0.010 to 0.060 of Ti and 0.010 to 0.050% of Nb.
 4. Product, according to claim 3, characterized in that it presents an area reduction value in the uniaxial tensile assay between 87% and 94%.
 5. Use of a hot rolled long steel product, as it is defined in claim 1 or 3, characterized in that it is for use in cold drawn wires.
 6. Use, according to claim 5, characterized in that it is for use in wire rods and bars.
 7. Use, according to claim 5, characterized in that it is for use in drawn bars and wires.
 8. Use, according to claim 5, characterized in that it is for use in pins, rods and fasteners selected from the group consisting of screws, nuts, nails, staples and rivets. 